Recently, microcomputers have been widely used for controlling various devices and apparatus. If a system to be controlled by a microcomputer is relatively small, Read Only Memories (ROMs) are often used as program memories for the microcomputer. When a ROM is used as a program memory of a microcomputer, a series of processes, such as a process from a program assembly to a program writing into a ROM, is executed relatively easily by using a development tool on the market.
However, none of the development tools on the market produces parity bits with respect to a control program prestored in the ROM. Accordingly, if it is desired to add parity bits to the ROM program memory, the software of the development tool must be changed. Furthermore, if the program is to be changed due to a bug which arises during debugging the program of the microcomputer, or due to a change of the application requirement of the microcomputer, the parity bits corresponding to the control program also must be altered.